Known units for gumming sheet material normally comprise mechanical gumming devices, in particular roller or spray devices, which involve several drawbacks.
In particular, roller gumming devices operate at relatively slow speed, by now well below the demands of modern machinery and in particular modern packing machines. Moreover, featuring a gumming roller rotating and partly immersed in a bath of gum, roller gumming devices are particularly "dirty" and require frequent cleaning. Finally, during machine stoppages, the bath of gum tends to dry by being left unstirred in contact with the air.
Spray gumming devices, on the other hand, are fairly expensive and unreliable due to frequent clocking of the spray nozzles.
FR-A1-981849 discloses a unit for gumming sheet material, wherein a roller advances the sheet material in contact with the surface of a bath of gum and an electric field is generated between the roller and the bath of gum to increase the portion of the sheet in contact with the glue. The aforementioned unit has several drawbacks; in particular, no possibility exists to gum, with such a unit, selected portions of the sheet material owing to the fact that, in such a unit, the sheet material is advanced in contact with the surface of the bath of gum. Furthermore, other portions of the sheet material, which have not to be gummed, may be soiled by the gum.